I bought a used 5-year old SRM Dura-Ace 7900 power meter. It's been used only about 500h by a pro cyclist few years ago.Anyway, when I first time did the zero offset, the value was 6500 +-10, so pretty stable, but the reading was insanely high!Well I didn't mind and went for a ride. The power seemed to be way too low. Went back home, did a calibration check with a 10kg weight and changed the slope from 19.6 to 16.1. After that I went out again, and the readings were somewhat normal. At least I think so. Did few sprints and the max power was about the same I knew I could do.

So do I have a problem here? Should I send it to SRM or just leave it like this _if_ it works well? Only one (or actually two) ride(s) done.

assuming it's using the ltc-7pn, according to the spec. sheet an isolated battery has c. 10-year shelf life, there must be more drain than that even in an idle srm so it's probably due for some new batteries and a calibration

When you have an SRM, you should be able to say more than "max power was about the same I knew I could do."I understand what you mean, but I'd send it in. Then you can check your own calibration method against the factory-calibration. It's worth it.

If you are mechanically inclined and know what digital VOM stands for, since the SRM is way out of warranty, I suggest that you pop off the back cover and measure the voltages of the batteries and compare to the spec printed on the label of the battery. If the measured voltage is more than .25-.35 volts below spec, time for new batteries.

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